Hexagonal strontium ferrite powder, magnetic recording medium, and magnetic recording and reproducing apparatus

ABSTRACT

A hexagonal strontium ferrite powder, in which an average particle size is 10.0 to 25.0 nm, a content of one or more kinds of atom selected from the group consisting of a gallium atom, a scandium atom, an indium atom, and an antimony atom is 1.0 to 15.0 atom % with respect to 100.0 atom % of an iron atom, and a coercivity Hc is greater than 2,000 Oe and smaller than 4,000 Oe. A magnetic recording medium including: a non-magnetic support; and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, in which the ferromagnetic powder is the hexagonal strontium ferrite powder. A magnetic recording and reproducing apparatus including this magnetic recording medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2019-003419 filed on Jan. 11, 2019. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a hexagonal strontium ferrite powder, amagnetic recording medium, and a magnetic recording and reproducingapparatus.

2. Description of the Related Art

A hexagonal ferrite powder is widely used as a ferromagnetic powder in amagnetic recording field and the like. In recent years, variousproposals regarding the hexagonal ferrite powder have been made forimproving properties thereof (see JP1995-093742A (JP-H07-093742A) andJP2007-126306A).

SUMMARY OF THE INVENTION

A crystal structure of a hexagonal ferrite includes at least an ironatom, a divalent metal atom, and an oxygen atom as constituting atoms.The hexagonal strontium ferrite powder has a crystal structure of ahexagonal ferrite in which a divalent metal atom mainly included is astrontium atom. It is thought that the hexagonal strontium ferritepowder is advantageous for increasing a reproducing output, in a case ofreproducing information recorded on a magnetic recording medium,compared to other kinds of the hexagonal ferrite powder.

In recent years, in the magnetic recording field, recording with higherdensity has proceeded and, along with this, a decrease in particle sizeof ferromagnetic powder (hereinafter, referred to as “atomization”) isrequired. However, the recording density is increased by the atomizationof the ferromagnetic powder, magnetization attenuation occurs due toreduction of a magnetic field. Therefore, in order to improveelectromagnetic conversion characteristics, it is desired to prevent themagnetization attenuation by increasing a coercivity Hc of theferromagnetic powder.

In consideration of these circumstances, the inventors have researchedthe atomization and an increase in coercivity of the hexagonal strontiumferrite powder. However, as a result of the research, it was clear thatonly the atomization and increase in coercivity are not enough forimproving the electromagnetic conversion characteristics.

One aspect of the invention provides for a hexagonal strontium ferritepowder capable of being used for manufacturing a magnetic recordingmedium having excellent electromagnetic conversion characteristics.

According to one aspect of the invention, there is provided a hexagonalstrontium ferrite powder, in which an average particle size is 10.0 to25.0 nm, a content of one or more kinds of atom selected from the groupconsisting of a gallium atom, a scandium atom, an indium atom, and anantimony atom is 1.0 to 15.0 atom % with respect to 100.0 atom % of aniron atom, and a coercivity Hc is greater than 2,000 Oe and smaller than4,000 Oe.

In regard to the unit Oe (oersted), 1 Oe=79.6 A (amperes). 2,000 Oe=159kA/m, and 4,000 Oe=318 kA/m.

In an embodiment, the hexagonal strontium ferrite powder may be aferromagnetic powder for magnetic recording.

In an embodiment, a mass magnetization σs of the hexagonal strontiumferrite powder may be equal to or greater than 41 A·m²/kg.

In an embodiment, the content of the one or more kinds of atom selectedfrom the group consisting of a gallium atom, a scandium atom, an indiumatom, and an antimony atom in the hexagonal strontium ferrite powder maybe 1.0 to 12.0 atom % with respect to 100.0 atom % of an iron atom.

In an embodiment, the hexagonal strontium ferrite powder may include agallium atom.

In an embodiment, the hexagonal strontium ferrite powder may include ascandium atom.

In an embodiment, the hexagonal strontium ferrite powder may include anindium atom.

In an embodiment, the hexagonal strontium ferrite powder may include anantimony atom.

In an embodiment, the hexagonal strontium ferrite powder may furtherinclude an aluminum atom.

In an embodiment, the hexagonal strontium ferrite powder may furtherinclude a neodymium atom.

In an embodiment, the hexagonal strontium ferrite powder may have amagnetoplumbite type crystal structure.

According to another aspect of the invention, there is provided amagnetic recording medium comprising: a non-magnetic support; and amagnetic layer including a ferromagnetic powder and a binding agent onthe non-magnetic support, in which the ferromagnetic powder is thehexagonal strontium ferrite powder.

According to still another aspect of the invention, there is provided amagnetic recording and reproducing apparatus comprising: the magneticrecording medium; and a magnetic head.

According to one aspect of the invention, it is possible to provide ahexagonal strontium ferrite powder usable for manufacturing a magneticrecording medium having excellent electromagnetic conversioncharacteristics. In addition, according to one aspect of the invention,it is possible to provide a magnetic recording medium including such ahexagonal strontium ferrite powder in a magnetic layer, and a magneticrecording and reproducing apparatus including this magnetic recordingmedium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hexagonal Strontium FerritePowder

In a hexagonal strontium ferrite powder according to one aspect of theinvention, an average particle size is 10.0 to 25.0 nm, a content of oneor more kinds of atom selected from the group consisting of a galliumatom, a scandium atom, an indium atom, and an antimony atom is 1.0 to15.0 atom % with respect to 100.0 atom % of an iron atom, and acoercivity Hc is greater than 2,000 Oe and smaller than 4,000 Oe. Thehexagonal strontium ferrite powder is suitable as a ferromagnetic powderfor magnetic recording and can be used for forming a magnetic layer of acoating type magnetic recording medium, for example.

Hereinafter, the hexagonal strontium ferrite powder will be describedmore specifically.

Average Particle Size

An average particle size of the hexagonal strontium ferrite powder is10.0 to 25.0 nm. The average particle size of the hexagonal strontiumferrite powder is equal to or smaller than 25.0 nm, preferably equal toor smaller than 23.0 nm, more preferably equal to or smaller than 21.0nm, and even more preferably equal to or smaller than 18.0 nm, from aviewpoint of realization of high density recording. The average particlesize of the hexagonal strontium ferrite powder is equal to or greaterthan 10.0 nm, preferably equal to or greater than 12.0 nm, and morepreferably equal to or greater than 14.0 nm, from a viewpoint ofstability of magnetization.

In the invention and the specification, average particle sizes ofvarious powder are values measured by the following method with atransmission electron microscope.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification of 500,000 to obtain animage of particles configuring the powder. A target particle is selectedfrom the obtained image of particles, an outline of the particle istraced with a digitizer, and a size of the particle (primary particle)is measured. The primary particle is an independent particle which isnot aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate are directly in contact with each other, but also includes anaspect in which a binding agent or an additive which will be describedlater is interposed between the particles.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph of 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter.

The equivalent circle diameter is a value obtained by a circleprojection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

Coercivity Hc

A coercivity Hc of the hexagonal strontium ferrite powder is greaterthan 2,000 Oe and smaller than 4,000 Oe. The coercivity Hc of thehexagonal strontium ferrite powder is greater than 2,000 Oe, preferablyequal to or greater than 2,150 Oe, more preferably equal to or greaterthan 2,300 Oe, and even more preferably equal to or greater than 2,300Oe, from a viewpoint of improving electromagnetic conversioncharacteristics. As the coercivity Hc is high, ease of recording(writing) of the information tends to be deteriorated, and accordingly,the electromagnetic conversion characteristics tend to be deteriorated.From this viewpoint, the coercivity Hc of the hexagonal strontiumferrite powder is smaller than 4,000 Oe, preferably equal to or smallerthan 3,800 Oe, and more preferably equal to or smaller than 3,500 Oe.2,150 Oe=171 kA/m, 2,300 Oe=183 kA/m, 2,500 Oe=199 kA/m, 3,000 Oe =239kA/m, 3,500 Oe=279 kA/m, 3,800 Oe=303 kA/m.

The coercivity Hc can be measured by a well-known measurement devicecapable of measuring magnetic properties such as an oscillation sampletype magnetic-flux meter. In the invention and the specification, thecoercivity Hc is a value measured at a measurement temperature of 25°C.±1° C. The measurement temperature is an atmosphere temperature of thesurroundings of the hexagonal strontium ferrite powder during themeasurement. The same applies to a mass magnetization σs which will bedescribed later.

Constituting Atom

The hexagonal strontium ferrite powder is one kind of hexagonal ferritepowder and has a crystal structure of hexagonal ferrite. A crystalstructure of hexagonal ferrite includes at least an iron atom, adivalent metal atom, and an oxygen atom as constituting atoms. Adivalent metal atom is a metal atom which can be divalent cations asions, and examples thereof include an alkali earth metal atom such as astrontium atom, a barium atom, or a calcium atom, and a lead atom. Inthe invention and the specification, the hexagonal strontium ferritepowder is powder in which main divalent metal atom included in thispowder is a strontium atom, and the main divalent metal atom is adivalent metal atom occupying the greatest content in the divalent metalatom included in the powder based on atom %. A content of the strontiumatom in the hexagonal strontium ferrite powder can be, for example, 2.0to 15.0 atom % with respect to 100.0 atom % of the iron atom. In oneaspect, in the hexagonal strontium ferrite powder, the divalent metalatom included in this powder can be only a strontium atom. In anotheraspect, the hexagonal strontium ferrite powder can also include one ormore kinds of other divalent metal atoms, in addition to the strontiumatom. For example, the hexagonal strontium ferrite powder can include abarium atom and/or a calcium atom, instead of the strontium atom. In acase where the barium atom and/or the calcium atom is included as thedivalent metal atom other than the strontium atom, a content of a bariumatom and a content of a calcium atom in the hexagonal strontium ferritepowder respectively can be, for example, 0.05 to 5.0 atom % with respectto 100.0 atom % of the iron atom. In the invention and thespecification, the divalent metal atom considered when defining thehexagonal strontium ferrite powder does not a rare-earth atom. The“rare-earth atom” in the invention and the specification is an atomselected from the group consisting of a scandium atom (Sc), a yttriumatom (Y), and a lanthanoid atom. The lanthanoid atom is selected fromthe group consisting of a lanthanum atom (La), a cerium atom (Ce), apraseodymium atom (Pr), a neodymium atom (Nd), a promethium atom (Pm), asamarium atom (Sm), a europium atom (Eu), a gadolinium atom (Gd), aterbium atom (Tb), a dysprosium atom (Dy), a holmium atom (Ho), anerbium atom (Er), a thulium atom (Tm), a ytterbium atom (Yb), and alutetium atom (Lu).

As the crystal structure of the hexagonal ferrite, a magnetoplumbitetype (also referred to as an “M type”), a W type, a Y type, and a Z typeare known. The hexagonal strontium ferrite powder may have any crystalstructure of hexagonal ferrite. The crystal structure can be confirmedby X-ray diffraction analysis. In the hexagonal strontium ferritepowder, a single crystal structure or two or more kinds of crystalstructure can be detected by the X-ray diffraction analysis. Forexample, in one aspect, in the hexagonal strontium ferrite powder, onlythe M type crystal structure can be detected by the X-ray diffractionanalysis. For example, the M type hexagonal ferrite is represented by acompositional formula of AFe₁₂O₁₉. Here, A represents a divalent metalatom, in a case where the hexagonal strontium ferrite powder has the Mtype, A is only a strontium atom (Sr), or in a case where a plurality ofdivalent metal atoms are included as A, the strontium atom (Sr) occupiesthe hexagonal strontium ferrite powder with the greatest content basedon atom % as described above. A content of the divalent metal atom inthe hexagonal ferrite powder is generally determined according to thetype of the crystal structure of the hexagonal ferrite and is notparticularly limited. The same applies to a content of an iron atom anda content of an oxygen atom. The hexagonal strontium ferrite powder atleast includes an iron atom, a strontium atom, an oxygen atom, and oneor more kinds of atom selected from the group consisting of a galliumatom (Ga), a scandium atom (Sc), an indium atom (In), and an antimonyatom (Sb), and may or may not include atoms other than these atoms.Hereinafter, one or more kinds of atom selected from the groupconsisting of a gallium atom (Ga), a scandium atom (Sc), an indium atom(In) and an antimony atom (Sb) are “M atoms”. The various M atoms aremetal atoms (trivalent metal atoms) which may become trivalent cationsand are not classified to divalent metal atoms considered when definingthe hexagonal strontium ferrite powder. The hexagonal strontium ferritepowder may include, as the M atom, only one kind of atom selected fromthe group consisting of a gallium atom, a scandium atom, an indium atom,and an antimony atom, may include two kinds thereof in any combination,and may include four kinds thereof in any combination. In a case wheretwo or more kinds of atoms are included as the M atoms, a content of theM atom below is obtained as a total content of the M atoms included.

The hexagonal strontium ferrite powder includes 1.0 to 15.0 atom % ofthe one or more kinds of atom selected from the group consisting of agallium atom, a scandium atom, an indium atom, and an antimony atom (Matoms) with respect to 100.0 atom % of the iron atom. The inventors havethought that the content of the M atoms described above may be a reasonwhy the hexagonal strontium ferrite powder having the average particlesize in the range described above and the coercivity Hc in the rangedescribed above can be used for manufacturing a magnetic recordingmedium having excellent electromagnetic conversion characteristics.Specifically, the inventors have surmised that the content of the Matoms in the range described above contributes to a decrease inswitching field distribution (SFD) (hereinafter, referred to as“realization of low SFD”). However, this is merely a surmise and theinvention is not limited thereto. The content of the M atoms in thehexagonal strontium ferrite powder is 1.0 atom %, preferably equal to orgreater than 1.5 atom %, more preferably equal to or greater than 2.0atom %, even more preferably equal to or greater than 2.5 atom %, andstill preferably equal to or greater than 3.0 atom %, with respect to100.0 atom % of the iron atom, from a viewpoint of improvingelectromagnetic conversion characteristics.

Meanwhile, it is considered that the hexagonal strontium ferrite powderis advantageous for increasing a reproducing output, in a case ofreproducing information recorded on a magnetic recording medium,compared to other kinds of hexagonal ferrite powders. The reasontherefor is because the mass magnetization as tends to be higher, thanthat of other kinds of hexagonal ferrite powder. The content of the Matom in the hexagonal strontium ferrite powder is equal to or smallerthan 15.0 atom % with respect to 100.0 atom % of the iron atom, from aviewpoint of improving electromagnetic conversion characteristics, andis preferably equal to or smaller than 14.0 atom %, more preferablyequal to or smaller than 13.0 atom %, even more preferably equal to orsmaller than 12.0 atom %, still preferably equal to or smaller than 11.0atom %, and still more preferably equal to or smaller than 10.0 atom %,from a viewpoint of preventing a decrease in the mass magnetization σs.

The hexagonal strontium ferrite powder includes the iron atom, thestrontium atom, the oxygen atom, and the M atom, and a content of theatoms other than these atoms can be, for example, equal to or smallerthan 10.0 atom %, can be 0 to 5.0 atom %, and may be 0 atom %, withrespect to 100.0 atom % of the iron atom. That is, in one aspect, thehexagonal strontium ferrite powder may not include atoms other than theiron atom, the strontium atom, the oxygen atom, and the M atom. In anembodiment, the hexagonal strontium ferrite powder may include atomsother than the iron atom, the strontium atom, the oxygen atom, and the Matom. As described above examples of such atoms include a barium atomand a calcium atom.

The content shown with atom % described above is obtained by convertinga value of the content (unit: % by mass) of each atom obtained bytotally dissolving the hexagonal strontium ferrite powder into a valueshown with atom % by using the atomic weight of each atom. In addition,in the invention and the specification, a given atom which is “notincluded” means that the content thereof obtained by performing totaldissolving and measurement by using an inductively coupled plasma (ICP)analysis device is 0% by mass. A detection limit of the ICP analysisdevice is generally equal to or smaller than 0.01 ppm (parts permillion) based on mass. The expression “not included” is used as ameaning including that a given atom is included with the amount smallerthan the detection limit of the ICP analysis device.

The total dissolving means dissolving performed until the powderremaining in the solution is not visually confirmed at the time of thecompletion of the dissolving.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 4 mol/L is held on ahot plate at a set temperature of 80° C. for 3 hours. The obtainedsolution is filtered with a membrane filter having a hole diameter of0.1 μm. The element analysis of the solution obtained as described aboveis performed by an ICP analysis device. By doing so, the content ofvarious atoms with respect to 100.0 atom % of the iron atom can beobtained. However, dissolving conditions of the total dissolving aremerely examples, and dissolving conditions capable of performing thetotal dissolving can be randomly used.

The hexagonal strontium ferrite powder includes an iron atom, astrontium atom, an oxygen atom, and one or more kinds of atom selectedfrom the group consisting of a gallium atom (Ga), a scandium atom (Sc),an indium atom (In), and an antimony atom (Sb), can randomly include abarium atom and/or a calcium atom, and can further randomly include oneor more kinds of the other atoms. Examples of the atoms which may befurther randomly included include an aluminum atom (Al), a neodymiumatom (Nd), a samarium atom (Sm), a yttrium atom (Y), and a dysprosiumatom (Dy). For example, in a case where the hexagonal strontium ferritepowder includes an aluminum atom, a content of the aluminum atom can be1.0 to 20.0 atom % and is preferably 2.0 to 15.0 atom % with respect to100.0 atom % of the iron atom. For example, in a case where thehexagonal strontium ferrite powder is a neodymium atom, a content of theneodymium atom can be 1.0 to 15.0 atom % and is preferably 3.0 to 12.0atom % with respect to 100.0 atom % of the iron atom. In a case wherethe content of the aluminum atom or the neodymium atom is increased, thecoercivity Hc tends to increase.

Mass Magnetization σs

From a viewpoint of increasing reproducing output in a case ofreproducing information recorded on a magnetic recording medium, it isdesirable that the mass magnetization as of ferromagnetic powderincluded in the magnetic recording medium is high. In an embodiment, themass magnetization σs of the hexagonal strontium ferrite powder can beequal to or greater than 41 A·m²/kg, is preferably equal to or greaterthan 42 A·m²/kg, more preferably equal to or greater than 43 A·m²/kg,even more preferably equal to or greater than 44 A·m²/kg, stillpreferably equal to or greater than 45 A·m²/kg, still more preferablyequal to or greater than 46 A·m²/kg, still even more preferably equal toor greater than 47 A·m²/kg, and still further more preferably equal toor greater than 48 A·m²/kg. On the other hand, from a viewpoint of noisereduction, σs is preferably equal to or smaller than 80 A·m²/kg and morepreferably equal to or smaller than 60 A·m²/kg. σs can be measured byusing a well-known measurement device capable of measuring magneticproperties such as an oscillation sample type magnetic-flux meter.

Manufacturing Method

The hexagonal strontium ferrite powder can be manufactured by awell-known manufacturing method as a manufacturing method of hexagonalferrite, for example, a glass crystallization method, a coprecipitationmethod, a reverse micelle method, or a hydrothermal synthesis method.Hereinafter, a manufacturing method using a glass crystallization methodwill be described as a specific aspect. However, the hexagonal strontiumferrite powder can be manufactured by a method other than the glasscrystallization method. As an example, for example, the hexagonalstrontium ferrite powder can also be manufactured by a hydrothermalsynthesis method. The hydrothermal synthesis method is a method ofheating an aqueous solution including a hexagonal strontium ferriteprecursor to convert the hexagonal strontium ferrite precursor intohexagonal strontium ferrite. Particularly, from a viewpoint of ease ofmanufacturing of the atomized hexagonal strontium ferrite powder havinga small particle size, a continuous hydrothermal synthesis method ofheating and pressurizing an aqueous solution including a hexagonalstrontium ferrite precursor while sending the aqueous solution to areaction flow path to convert the hexagonal strontium ferrite precursorinto hexagonal strontium ferrite by using high reactivity of the heatedand pressurized water, preferably water in a subcritical tosupercritical state is preferable.

Manufacturing Method Using Glass Crystallization Method

The glass crystallization method generally includes the following steps.

(1) Step of melting a raw material mixture at least including ahexagonal strontium ferrite formation component and a glass formationcomponent to obtain a molten material (melting step);

(2) Step of rapidly cooling the molten material to obtain an amorphousmaterial (non-crystallization step);

(3) Step of heating the amorphous material and obtaining a crystallinematerial including hexagonal strontium ferrite particles andcrystallized glass component precipitated by the heating(crystallization step); and

(4) Step of collecting the hexagonal strontium ferrite particles fromthe crystalline material (particle collecting step).

Hereinafter, the step will be described later more specifically.

Melting Step

The raw material mixture used in the glass crystallization method forobtaining the hexagonal strontium ferrite powder includes the hexagonalstrontium ferrite formation component and the glass formation component.The glass formation component here is a component which may show a glasstransition phenomenon and may be subjected to non-crystallization(vitrification), and in a general glass crystallization method, a B₂O₃component is used. Even in a case of using the glass crystallizationmethod for obtaining the hexagonal strontium ferrite powder, a B₂O₃component as the glass formation component, can be used. Each componentincluded in the raw material mixture in the glass crystallization methodis present as oxide or as various salt which may change into oxideduring the step such as melting. The “B₂O₃ component” in the inventionand the specification include B₂O₃ as it is, and various salts such asH₃BO₃ which may change to B₂O₃ during the step. The same applies toother components.

As the hexagonal strontium ferrite formation component included in theraw material mixture, oxide including an atom which is a constitutingatom of the crystal structure of hexagonal strontium ferrite can beused. As specific examples, a Fe₂O₃ component, and a SrO component, andthe like are used. In addition, in order to obtain hexagonal strontiumferrite powder including a barium atom, a BaO component can be used, andin order to obtain hexagonal strontium ferrite powder including calciumatom, a CaO component can be used.

In order to obtain hexagonal strontium ferrite powder including an Matom, an oxide component of the M atom (for example, Ga₂O₃, Sc₂O₃,Ir₂O₃, or Sb₂O₃) is used. In addition, for example, in order to obtainhexagonal strontium ferrite powder including an aluminum atom, an Al₂O₃component is used, and in order to obtain hexagonal strontium ferritepowder including a neodymium atom, a Nd₂O₃ component is used.

A content of each component in the raw material mixture is notparticularly limited, and may be determined according to the compositionof the hexagonal strontium ferrite powder to be obtained. The rawmaterial mixture can be prepared by weighing and mixing variouscomponents. Then, the raw material mixture is melted and a moltenmaterial is obtained. A melting temperature may be set according to thecomposition of the raw material mixture, and is generally 1,000° C. to1,500° C. A melting time may be suitably set so that the raw materialmixture is sufficiently melted.

Non-Crystallization Step

Next, the obtained molten material is rapidly cooled to obtain anamorphous material. The rapid cooling can be performed in the samemanner as in a rapid cooling generally performed for obtaining anamorphous material in the glass crystallization method, and the rapidcooling step can be performed, for example, by a well-known method suchas a method of pouring the molten material on a rapidly rotatedwater-cooled twin roller and performing rolling and rapid cooling.

Crystallization Step

After the rapid cooling, the obtained amorphous material is heated. Bythe heating, the hexagonal strontium ferrite particles and crystallizedglass component can be precipitated. A particle size of the precipitatedhexagonal strontium ferrite particles can be controlled depending onheating conditions. In a case where a heating temperature(crystallization temperature) for crystallization increases, a particlesize of the hexagonal strontium ferrite particles to be precipitatedtends to increase. In addition, a crystallization temperature increases,the coercivity Hc of the hexagonal strontium ferrite powder to beprecipitated tends to increase. By considering the above point, it ispreferable to control the heating conditions, so as to obtain thehexagonal strontium ferrite powder having average particle size and thecoercivity Hc in the ranges described above. In an embodiment, thecrystallization temperature is preferably 600° C. to 700° C. Inaddition, In an embodiment, the heating time for crystallization(holding time at the crystallization temperature) is, for example, 0.1to 24 hours and preferably 0.15 to 8 hours. Further, In an embodiment, arate of temperature increase until the temperature achieves thecrystallization temperature is preferably 1.0 to 10.0 ° C./min, morepreferably 1.5 to 7.0 ° C./min, and even more preferably 2.0 to 5.0 °C./min.

Particle Collecting Step

The crystalline material obtained by heating the amorphous materialincludes the hexagonal strontium ferrite particles and the crystallizedglass component. Therefore, in a case of performing acid treatment withrespect to the crystalline material, the crystallized glass componentsurrounding the hexagonal strontium ferrite particles is dissolved andremoved, thereby collecting the hexagonal strontium ferrite particles.Before the acid treatment, it is preferable to perform coarse crushingfor increasing efficiency of the acid treatment. The coarse crushing maybe performed by a dry or wet method. The coarse crushing conditions canbe set according to a well-known method. The acid treatment forcollecting particles can be performed by a method generally performed inthe glass crystallization method such as acid treatment after heating.After that, by performing post-treatment such as water washing ordrying, if necessary, the hexagonal strontium ferrite particles can beobtained.

Hereinabove, the manufacturing method of the hexagonal strontium ferritepowder according to one aspect of the invention has been described.However, the hexagonal strontium ferrite powder according to one aspectof the invention is not limited to hexagonal strontium ferrite powdermanufactured by the specific aspect.

Magnetic Recording Medium

One aspect of the invention relates to a magnetic recording mediumincluding a non-magnetic support; and a magnetic layer including aferromagnetic powder and a binding agent on the non-magnetic support, inwhich the ferromagnetic powder is the hexagonal strontium ferritepowder.

Hereinafter, the magnetic recording medium will be described morespecifically.

Magnetic Layer Ferromagnetic Powder

The details of the ferromagnetic powder included in the magnetic layerof the magnetic recording medium are as described above. The content(filling percentage) of the ferromagnetic powder in the magnetic layeris preferably 50% to 90% by mass and more preferably 60% to 90% by mass.The components other than the ferromagnetic powder in the magnetic layerare at least a binding agent and one or more kinds of additives may berandomly included. A high filling percentage of the ferromagnetic powderin the magnetic layer is preferable from a viewpoint of improvementrecording density.

Binding Agent and Curing Agent

The magnetic layer includes a binding agent together with theferromagnetic powder. As the binding agent, one or more kinds of resinis used. The resin may be a homopolymer or a copolymer. As the bindingagent included in the magnetic layer, a resin selected from apolyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in a non-magnetic layer and/or a back coating layerwhich will be described later. the binding agent described above,description disclosed in paragraphs 0029 to 0031 of JP2010-024113A canbe referred to. An average molecular weight of the resin used as thebinding agent can be, for example, 10,000 to 200,000 as a weight-averagemolecular weight. The weight-average molecular weight of the inventionand the specification is a value obtained by performing polystyreneconversion of a value measured by gel permeation chromatography (GPC).As the measurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with a resin whichcan be used as the binding agent. As the curing agent, in one aspect, athermosetting compound which is a compound in which a curing reaction(crosslinking reaction) proceeds due to heating can be used, and inanother aspect, a photocurable compound in which a curing reaction(crosslinking reaction) proceeds due to light irradiation can be used.At least a part of the curing agent is included in the magnetic layer ina state of being reacted (crosslinked) with other components such as thebinding agent, by proceeding the curing reaction in the magnetic layerforming step. In a case where a composition used for forming the otherlayer includes the curing agent, this point also identically applies tothe layer formed using this composition. The preferred curing agent is athermosetting compound, polyisocyanate is suitable. For details of thepolyisocyanate, descriptions disclosed in paragraphs 0124 and 0125 ofJP2011-216149A can be referred to, for example. The amount of the curingagent in the magnetic layer forming composition can be, for example, 0to 80.0 parts by mass, and is preferably 50.0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent.

Additives

The magnetic layer includes ferromagnetic powder and the binding agent,and may include one or more kinds of additives, if necessary. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude non-magnetic powder (for example, inorganic powder or carbonblack), a lubricant, a dispersing agent, a dispersing assistant, anantibacterial agent, an antistatic agent, and an antioxidant. Forexample, for the lubricant, a description disclosed in paragraphs 0030to 0033, 0035, and 0036 of JP2016-126817A can be referred to. Thelubricant may be included in a non-magnetic layer which will bedescribed later. For the lubricant which can be included in thenon-magnetic layer, a description disclosed in paragraphs 0030, 0031,and 0034 to 0036 of JP2016-126817A can be referred to. For thedispersing agent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837A can be referred to. The dispersing agent may be includedin the non-magnetic layer forming composition. For the dispersing agentwhich can be included in the non-magnetic layer forming composition, adescription disclosed in paragraph 0061 of JP2012-133837A can bereferred to. In addition, as the non-magnetic powder which may beincluded in the magnetic layer, non-magnetic powder which can functionas an abrasive, non-magnetic powder (for example, non-magnetic colloidparticles) which can function as a projection formation agent whichforms projections suitably protruded from the surface of the magneticlayer, and the like can be used. As the additives, a commerciallyavailable product can be suitably selected according to the desiredproperties or manufactured by a well-known method, and can be used withany amount.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recordingmedium may include a magnetic layer directly on a non-magnetic support,or may include a non-magnetic layer including non-magnetic powder and abinding agent between the non-magnetic support and the magnetic layer.The non-magnetic powder used in the non-magnetic layer may be powder ofan inorganic substance (inorganic powder) or powder of an organicsubstance (organic powder). In addition, carbon black and the like canbe used. Examples of the inorganic substance include metal, metal oxide,metal carbonate, metal sulfate, metal nitride, metal carbide, and metalsulfide. These non-magnetic powder can be purchased as a commerciallyavailable product or can be manufactured by a well-known method. Fordetails thereof, descriptions disclosed in paragraphs 0146 to 0150 ofJP2011-216149A can be referred to. For carbon black which can be used inthe non-magnetic layer, descriptions disclosed in paragraphs 0040 and0041 of JP2010-024113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the magnetic recording medium also includes asubstantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

As the non-magnetic support (hereinafter, also simply referred to as a“support”), well-known components such as polyethylene terephthalate,polyethylene naphthalate, polyamide, polyamide imide, aromatic polyamidesubjected to biaxial stretching are used. Among these, polyethyleneterephthalate, polyethylene naphthalate, and polyamide are preferable.Corona discharge, plasma treatment, easy-bonding treatment, or heattreatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic recording medium can also include a back coating layerincluding non-magnetic powder and a binding agent on a surface side ofthe non-magnetic support opposite to the surface side provided with themagnetic layer. The back coating layer preferably includes one or bothof carbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can berandomly included in the back coating layer, a well-known technologyregarding the back coating layer can be applied, and a well-knowntechnology regarding the treatment of the magnetic layer and/or thenon-magnetic layer can also be applied. For example, for the backcoating layer, descriptions disclosed in paragraphs 0018 to 0020 ofJP2006-331625A and page 4, line 65, to page 5, line 38, of U.S. Pat. No.7,029,774 can be referred to.

Thicknesses of Non-Magnetic Support and Each Layer

Regarding thicknesses of the non-magnetic support and each layer, athickness of the non-magnetic support is, for example, 3.0 to 80.0 μm,preferably 3.0 to 20.0 μm and more preferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of a magnetic head used, a head gaplength, a recording signal band, and the like. The thickness of themagnetic layer is generally 10 to 150 nm, preferably 20 to 120 nm andmore preferably 30 to 100 nm, from a viewpoint of realization ofhigh-density recording. The magnetic layer may be at least one layer, orthe magnetic layer can be separated to two or more layers havingmagnetic properties, and a configuration regarding a well-knownmultilayered magnetic layer can be applied. In a case of themultilayered magnetic layer, the thickness of the magnetic layer is atotal thickness of the plurality of magnetic layers.

A thickness of the non-magnetic layer is, for example, 0.05 to 3.0 μm,preferably 0.05 to 2.0 μm, and even more preferably 0.05 to 1.5 μm.

The thickness of the back coating layer is preferably equal to orsmaller than 0.9 μm and more preferably 0.1 to 0.7 μm.

The thicknesses of each layer and the non-magnetic support of themagnetic recording medium can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is, for example, exposed by awell-known method of ion beams or microtome, and the exposed crosssection is observed with a scanning electron microscope. In the crosssection observation, various thicknesses can be acquired as a thicknessacquired at one portion of the cross section, or an arithmetical mean ofthicknesses acquired at a plurality of portions of two or more portions,for example, two portions which are randomly extracted. In addition, thethickness of each layer may be determined as a designed thicknesscalculated according to the manufacturing conditions.

Manufacturing Method of Magnetic Recording Medium

A step of manufacturing a composition for forming the magnetic layer,the non-magnetic layer, or the back coating layer generally includes atleast a kneading step, a dispersing step, and a mixing step which isprovided before or after these steps, if necessary. Each step may bedivided into two or more stages. Various components may be added at aninitial stage or in a middle stage of each step.

In addition, each component may be separately added in two or moresteps.In order to manufacture the magnetic recording medium, a well-knownmanufacturing technology of the related art can be used in a part of thestep or in the entire step.For example, in the kneading step, an open kneader, a continuouskneader, a pressure kneader, or a kneader having a strong kneading forcesuch as an extruder is preferably used.For the details of these kneading processes, descriptions disclosed inJP1989-106338A (JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) canbe referred to.In order to disperse the composition for forming each layer, glass beadscan be used as dispersion beads. As the dispersion beads, zirconiabeads, titania beads, and steel beads which are dispersion beads havinghigh specific gravity are suitable. These dispersion beads arepreferably used by optimizing a particle diameter (bead diameter) and afilling percentage of these dispersion beads. As a dispersing machine, awell-known dispersing machine can be used. Each layer formingcomposition may be filtered by a well-known method before performing thecoating step. The filtering can be performed by using a filter, forexample. As the filter used in the filtering, a filter having a holediameter of 0.01 to 3 μm (for example, a glass fiber filter or apolypropylene filter) can be used, for example.

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the non-magnetic support or performingmultilayer coating with the non-magnetic layer forming composition inorder or at the same time. In the aspect of performing the alignmentprocess, while the coating layer of the magnetic layer formingcomposition is wet, an alignment process is performed on the coatinglayer in an alignment zone. For the alignment process, variouswell-known technologies disclosed in a paragraph 0052 of JP2010-024113Acan be referred to. For example, a homeotropic alignment process can beperformed by a well-known method such as a method using a polar opposingmagnet. In the alignment zone, a drying speed of the coating layer canbe controlled by a temperature of dry air, an air flow and/or atransportation speed in the alignment zone. In addition, the coatinglayer may be preliminarily dried before transporting in the alignmentzone. The back coating layer can be formed by applying the back coatinglayer forming composition to the side of the non-magnetic supportopposite to a side provided with the magnetic layer (or to be providedwith the magnetic layer). For the details of the manufacturing method ofthe magnetic recording medium, description disclosed in paragraphs 0051to 0057 of JP2010-024113A can also be referred to.

The magnetic recording medium according to one aspect of the inventiondescribed above can be a tape-shaped magnetic recording medium (magnetictape) in one aspect, and can be a disk-shaped magnetic recording medium(magnetic disk) in another aspect. For example, the magnetic tape isnormally used to be accommodated and circulated in a magnetic tapecartridge. A servo pattern can also be formed in the magnetic tape by awell-known method, in order to allow head tracking servo to be performedin the magnetic recording and reproducing apparatus. For example, theformation of the servo pattern can be performed on direct current(DC)-demagnetized magnetic layer. A direction of the demagnetization canbe a longitudinal direction or a vertical direction of the magnetictape. A direction of the magnetization, in a case of forming the servopattern (that is, magnetization region) can be a longitudinal directionor a vertical direction of the magnetic tape. The magnetic recordingmedium includes the hexagonal strontium ferrite powder according to oneaspect of the invention in the magnetic layer, and thus, it is possibleto exhibit excellent electromagnetic conversion characteristics. Themagnetic recording medium can be suitably used in a contact sliding typemagnetic recording and reproducing system in which a surface of themagnetic layer and a magnetic head come into contact with each other andslide thereon, in a case of performing recording and/or reproducing ofinformation.

Magnetic Recording and Reproducing Apparatus

One aspect of the invention relates to a magnetic recording andreproducing apparatus including the magnetic recording medium, and amagnetic head.

In the invention and the specification, the “magnetic recording andreproducing apparatus” means a device capable of performing at least oneof the recording of information on the magnetic recording medium or thereproducing of information recorded on the magnetic recording medium.Such a device is generally called a drive. The magnetic recording andreproducing apparatus can be a sliding type magnetic recording andreproducing apparatus. The magnetic head included in the magneticrecording and reproducing apparatus can be a recording head capable ofperforming the recording of information on the magnetic recordingmedium, or can be a reproducing head capable of performing thereproducing of information recorded on the magnetic recording medium. Inaddition, In an embodiment, the magnetic recording and reproducingapparatus can include both of a recording head and a reproducing head asseparate magnetic heads. In another aspect, the magnetic head includedin the magnetic recording and reproducing apparatus can also have aconfiguration of including both of a recording element and a reproducingelement in one magnetic head. As the reproducing head, a magnetic head(MR head) including a magnetoresistive (MR) type element as areproducing element which can read the information recorded on themagnetic recording medium with high sensitivity is preferable. As the MRhead, various well-known MR heads such as an AnisotropicMagnetoresistive(AMR) head, a Giant Magnetoresistive (GMR) head, and a TunnelMagnetoresistive (TMR) can be used. In addition, the magnetic head whichperforms the recording of information and/or the reproducing ofinformation may include a servo pattern reading element. Alternatively,as a head other than the magnetic head which performs the recording ofinformation and/or the reproducing of information, a magnetic head(servo head) including a servo pattern reading element may be includedin the magnetic recording and reproducing apparatus.

In the magnetic recording and reproducing apparatus, the recording ofinformation on the magnetic recording medium and/or the reproducing ofinformation recorded on the magnetic recording medium can be performedby bringing the surface of the magnetic layer of the magnetic recordingmedium into contact with the magnetic head and sliding, for example. Themagnetic recording and reproducing apparatus may include such a magneticrecording medium according to one aspect of the invention, andwell-known technologies can be applied for the other configurations.

EXAMPLES

Hereinafter, the invention will be described with reference to examplesmore specifically. However, the invention is not limited to aspectsshown in the examples. “Parts” and “%” in the following descriptionindicate “parts by mass” and “% by mass”, unless otherwise noted. “eq”is an equivalent which is a unit which cannot be converted into the SIunit. In addition, steps and evaluations described below are performedin an atmosphere at 23° C.±1° C., unless otherwise noted.

1. Preparation and Evaluation of Hexagonal Strontium Ferrite Powder (1)Preparation and Hexagonal Strontium Ferrite Powder Example 1

1,070 g of SrCO₃, 450 g of H₃BO₃, 675 g of Fe₂O₃, 64 g of Ga₂O₃, 29 g ofAl(OH)₃, 42 g of BaCO₃, and 21 g of CaCO₃ were weighed and mixed with amixer to obtain a raw material mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1,390° C., a tap hole provided on the bottom ofthe platinum crucible was heated while stirring the melted liquid, andthe melted liquid was extracted in a rod shape at approximately 6 g/sec.The extracted liquid was rolled and rapidly cooled with a water-cooledtwin roller to manufacture an amorphous material.

280 g of the manufactured amorphous material was put into an electricfurnace and heated to a temperature shown in Table 1 (crystallizationtemperature) at a rate of temperature increase of 3.5° C./min, and heldat the same temperature for 5 hours, to precipitate (crystallize)hexagonal strontium ferrite particles.

Then, a crystalline material obtained above including the hexagonalstrontium ferrite particles was coarsely crushed with a mortar. Adispersion process was performed with a paint shaker for 3 hours, byadding 1,000 g of zirconia beads having a particle diameter of 1 mm and800 ml of acetic acid having a concentration of 1% in a glass bottleincluding the obtained coarsely crushed material. After that, theobtained dispersion liquid was separated from the beads and put into astainless steel beaker. A dissolving process of the glass component wasperformed by leaving the dispersion liquid at a liquid temperature of100° C. for 3 hours, the precipitation was performed with a centrifugalseparator, decantation was repeated for washing, and the resultantmaterial was dried in a heating furnace at a temperature in the furnaceof 110° C. for 6 hours, thereby obtaining hexagonal strontium ferritepowder.

Comparative Example 1

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the amount Ga₂O₃ used in the preparation of theraw material mixture was set as 6.7 g.

Example 2

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the amount Ga₂O₃ used in the preparation of theraw material mixture was set as 11 g.

Example 3

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the amount Ga₂O₃ used in the preparation of theraw material mixture was set as 79 g.

Example 4

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the amount Ga₂O₃ used in the preparation of theraw material mixture was set as 110 g.

Comparative Example 2

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the amount Ga₂O₃ used in the preparation of theraw material mixture was set as 116 g.

Comparative Example 3

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperature shown in Table 1.

Example 5

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperature shown in Table 1.

Example 6

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperatures shown in Table 1.

Example 7

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperatures shown in Table 1.

Example 8

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperatures shown in Table 1.

Comparative Example 4

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperatures shown in Table 1.

Comparative Example 5

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,044 g of SrCO₃, 469 g of H₃BO₃, 675 g of Fe₂O₃,66 g of Ga₂O₃, and 140 g of BaCO₃ were weighed in the preparation of theraw material mixture, the raw material mixture was obtained by mixingthe components with each other, and the crystallization temperature waschanged to a temperature shown in Table 1.

Example 9

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,048 g of SrCO₃, 469 g of H₃BO₃, 675 g of Fe₂O₃,66 g of Ga₂O₃, and 133 g of BaCO₃ were weighed in the preparation of theraw material mixture, and the raw material mixture was obtained bymixing the components with each other.

Example 10

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,055 g of SrCO₃, 424 g of H₃BO₃, 678 g of Fe₂O₃,64 g of Ga₂O₃, 72 g of Al(OH)₃, 42 g of BaCO₃, and 21 g of CaCO₃ wereweighed in the preparation of the raw material mixture, the raw materialmixture was obtained by mixing the components with each other, and thecrystallization temperature was changed to a temperature shown in Table1.

Comparative Example 6

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,020 g of SrCO₃, 375 g of H₃BO₃, 678 g of Fe₂O₃,64 g of Ga₂O₃, 151 g of Al(OH)3, 42 g of BaCO₃, and 22 g of CaCO₃ wereweighed in the preparation of the raw material mixture, the raw materialmixture was obtained by mixing the components with each other, and thecrystallization temperature was changed to a temperature shown in Table1.

Example 11

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,054 g of SrCO₃, 424 g of H₃BO₃, 677 g of Fe₂O₃,64 g of Ga₂O₃, 71 g of Al(OH)₃, 42 g of BaCO₃, 21 g of CaCO₃, and 143 gof Nd₂O₃ were weighed in the preparation of the raw material mixture,the raw material mixture was obtained by mixing the components with eachother, and the crystallization temperature was changed to a temperatureshown in Table 1.

Example 12

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that the crystallization temperature was changed to atemperature shown in Table 1.

Example 13

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,073 g of SrCO₃, 450 g of H₃BO₃, 677 g of Fe₂O₃,47 g of Sc₂O₃, 29 g of Al(OH)3, 42 g of BaCO₃, and 21 g of CaCO₃ wereweighed in the preparation of the raw material mixture, and the rawmaterial mixture was obtained by mixing the components with each other.

Example 14

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 13, except that the amount of Sc₂O₃ used in the preparation ofthe raw material mixture was set as 7 g.

Example 15

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 13, except that the amount of Sc₂O₃ used in the preparation ofthe raw material mixture was set as 81 g.

Example 16

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,073 g of SrCO₃, 450 g of H₃BO₃, 677 g of Fe₂O₃,95 g of In₂O₃, 29 g of Al(OH)₃, 42 g of BaCO₃, and 21 g of CaCO₃ wereweighed in the preparation of the raw material mixture, and the rawmaterial mixture was obtained by mixing the components with each other.

Example 17

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 16, except that the amount of In₂O₃ used in the preparation ofthe raw material mixture was set as 15 g.

Example 18

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 16, except that the amount of In₂O₃ used in the preparation ofthe raw material mixture was set as 162 g.

Example 19

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,073 g of SrCO₃, 450 g of H₃BO₃, 677 g of Fe₂O₃,99 g of Sb₂O₃, 29 g of Al(OH)₃, 42 g of BaCO₃, and 21 g of CaCO₃ wereweighed in the preparation of the raw material mixture, and the rawmaterial mixture was obtained by mixing the components with each other.

Example 20

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 19, except that the amount of Sb₂O₃ used in the preparation ofthe raw material mixture was set as 15 g.

Example 21

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 19, except that the amount of Sb₂O₃ used in the preparation ofthe raw material mixture was set as 172 g.

Example 22

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,136 g of SrCO₃, 451 g of H₃BO₃, 677 g of Fe₂O₃,64 g of Ga₂O₃, and 29 g of Al(OH)₃ were weighed in the preparation ofthe raw material mixture, the raw material mixture was obtained bymixing the components with each other, and the crystallizationtemperature was changed to a temperature shown in Table 1.

Example 23

Hexagonal strontium ferrite powder was obtained in the same manner as inExample 1, except that 1,085 g of SrCO₃, 470 g of H₃BO₃, 677 g of Fe₂O₃,64 g of Ga₂O₃, 42 g of BaCO₃, and 21 g of CaCO₃ were weighed in thepreparation of the raw material mixture, the raw material mixture wasobtained by mixing the components with each other, and thecrystallization temperature was changed to a temperature shown in Table1.

(2) Evaluation of Hexagonal Strontium Ferrite Powder (X-Ray DiffractionAnalysis)

Sample powder was collected from the powder obtained in the examples andthe comparative examples, and the X-ray diffraction analysis wasperformed. As a result of analysis, all of the powder obtained in theexamples and the comparative examples showed a crystal structure ofmagnetoplumbite type (M type) hexagonal ferrite. In addition, a crystalphase detected by the X-ray diffraction analysis was a magnetoplumbitetype single phase. The X-ray diffraction analysis was performed byscanning with a CuKα ray at a voltage of 45 kV and intensity of 40 mAand by measuring X-ray diffraction pattern under the conditions.

PANalytical X'Pert Pro diffractometer, PIXcel detector

Soller slit of incident beam and diffraction beam: 0.017 radians

Fixed angle of dispersion slit: ¼ degrees

Mask: 10 mm

Scattering prevention slit: ¼ degrees

Measurement mode: continuous

Measurement time per 1 stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degrees

Contents of Various Atoms

12 mg of sample powder was collected from each hexagonal strontiumferrite powder of the examples and the comparative examples, elementanalysis of filtrate obtained by totally dissolving the sample powderunder the dissolving conditions described above was performed by the ICPanalysis device, and the contents of various atoms were obtained.

Average Particle Size

Sample powder was collected from each hexagonal strontium ferrite powderof the examples and the comparative examples, and an average particlesize was obtained by the method described above.

Coercivity Hc and Mass Magnetization σs

The coercivity Hc and the mass magnetization as of each hexagonalstrontium ferrite powder of the examples and the comparative exampleswere measured by using an oscillation sample type magnetic-flux meter(manufactured by Toei Industry Co., Ltd.)at a magnetic field strength of1,194 kA/m (15 kOe) at a measurement temperature of 25° C.±1° C.

2. Manufacturing and Evaluation of Magnetic Recording Medium (MagneticTape) (1) Manufacturing of Magnetic Recording Medium (Magnetic Tape)

A magnetic tape was manufactured by the following method by using eachhexagonal strontium ferrite powder of the examples and the comparativeexamples. Hereinafter, the magnetic tape manufactured by using thehexagonal strontium ferrite powder of Example 1 is referred to as amagnetic tape of Example 1. The same applies to the other examples andcomparative examples.

List of Magnetic Layer Forming Composition

Hexagonal strontium ferrite powder of the examples and the comparativeexamples: 100.0 parts

Polyurethane resin: 12.2 parts

-   -   Weight-average molecular weight: 10,000    -   Sulfonic acid group content: 0.5 meq/g

Diamond particles: 1.85 parts

-   -   Average particle size: 50 nm

Carbon black (#55 manufactured by Asahi Carbon Co., Ltd.): 0.5 parts

-   -   Average particle size: 0.015 μm

Stearic acid: 0.5 parts

Butyl stearate: 2.1 parts

Methyl ethyl ketone: 180.0 parts

Cyclohexanone: 100.0 parts

List of Non-Magnetic Layer Forming Composition

Non-magnetic powder α-iron oxide: 103.0 parts

-   -   Average particle size: 0.09 μm    -   BET (Brunauer-Emmett-Teller) specific surface area: 50 m²/g    -   pH: 7    -   Dibutyl phthalate (DBP) oil absorption amount: 27 to 38 g/100 g    -   Surface treating agent: Al₂O₃ (8% by mass)

Carbon black (CONDUCTEX TEX SC-U manufactured by Columbia Carbon): 25.0parts

A vinyl chloride copolymer (MR 104 manufactured by Zeon Corporation):12.9 parts

A polyurethane resin (UR8200 manufactured by Toyobo Co., Ltd.): 5.2parts

Phenylphosphonic acid: 3.5 parts

Butyl stearate: 1.1 parts

Stearic acid: 2.1 parts

Methyl ethyl ketone: 205.0 parts

Cyclohexanone: 135.0 parts

List of Back Coating Layer Forming Composition

Non-magnetic powder α-iron oxide: 80.0 parts

-   -   Average particle size: 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Cyclohexanone: 155.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 200.0 parts

Manufacturing of Magnetic Tape

Regarding each of the magnetic layer forming composition and thenon-magnetic layer forming composition, various components were kneadedwith a kneader. The component was transferred to a transverse sand millcontaining zirconia beads having a particle diameter of 1.0 mm by thefilling amount which is 65 volume % with respect to a volume of adispersion portion, and dispersed at 2,000 revolution per minutes (rpm)for 120 minutes (time for which the component is substantially held inthe dispersion portion). Regarding the magnetic layer formingcomposition, the obtained dispersion liquid was filtered by using afilter having a hole diameter of 1μm, thereby obtaining the magneticlayer forming composition. Regarding the non-magnetic layer formingcomposition, 6.5 parts of polyisocyanate and 7.0 parts of methyl ethylketone were added to the dispersion liquid obtained by the dispersionand filtered by using a filter having a hole diameter of 1 μm, therebyobtaining the magnetic layer forming composition.

A back coating layer forming composition was prepared by the followingmethod. The components excluding the lubricant (stearic acid and butylstearate), polyisocyanate, and 200.0 parts of cyclohexanone were kneadedby an open kneader and diluted, and was subjected to a dispersionprocess of 12 passes, with a transverse beads mill dispersing machineand zirconia beads having a particle diameter of 1.0 mm, by setting abead filling percentage as 80 volume %, a circumferential speed of rotordistal end as 10 m/sec, and a retention time for 1 pass as 2 minutes.After that, the remaining components were added to the dispersion liquidand stirred with a dissolver. The obtained dispersion liquid wasfiltered with a filter having an average hole diameter of 1 μm and theback coating layer forming composition was obtained.

After that, the non-magnetic layer forming composition was applied ontoone surface of a non-magnetic support made of polyethylene naphthalatehaving a thickness of 5.0 μm so that a thickness after the dryingbecomes 0.1 μm and was dried, and then, the non-magnetic layer wasformed.

Then, the magnetic layer forming composition was applied onto thenon-magnetic layer so that a thickness after the drying becomes 70 nm,and a coating layer was formed. A homeotropic alignment process wasperformed by applying a magnetic field having a magnetic field strengthof 0.6 T in a vertical direction with respect to a surface of thecoating layer, while the coating layer is wet, and then, the coatingsurface was dried to form a magnetic layer.

After that, the back coating layer forming composition was applied tothe opposite surface of the non-magnetic support so that a thicknessafter the drying becomes 0.4 μm and was dried, thereby forming the backcoating layer.

Then, a surface smoothing treatment (calender process) was performed bya calender configured of only a metal roll, at a surface temperature ofa calender roll of 90° C. and linear pressure of 300 kg/cm (294 kN/m).After that, slitting was performed to have a width of ½ inches (0.0127meters), and surface polishing treatment was performed, therebyobtaining a magnetic tape.

(2) Evaluation of Magnetic Recording Medium (Magnetic Tape) Evaluationof Electromagnetic Conversion Characteristics (Noise)

A magnetic signal was recorded on each magnetic tape of the examples andthe comparative examples in a tape longitudinal direction under thefollowing conditions and reproduced with a magnetoresistive (MR) head. Areproduction signal was frequency-analyzed with a spectrum analyzermanufactured by Shibasoku Co., Ltd. and the noise accumulated in therange of 0 to 600 kfci were evaluated. The unit, kfci, is a unit oflinear recording density (not able to be converted into the SI unitsystem).

Recording and Reproduction Conditions

Recording: recording track width 5 μm

-   -   Recording gap 0.17 μm    -   Head saturated magnetic flux density Bs 1.8 T

Reproduction: Reproduction track width 0.4 μm

-   -   Distance between shields (sh-sh distance) 0.08 μm

Evaluation Standard

5: Substantially no noise, a signal is excellent, and no error isobserved.

4: A degree of noise is smaller than that observed in the level 3. Asignal is excellent.

3: Noise is observed. A signal is excellent.

2: A degree of noise is greater than that observed in the level 3. Asignal is unclear.

1: Noise and signal cannot be distinguished or cannot be recorded.

The results of the above evaluation are shown in Table 1 (Tables 1-1 and1-2).

TABLE 1-1 Content of each constituent atom (with respect to 100.0 atom %of Fe atom) Kind of M atom Sr atom Ba atom Ca atom Al atom Nd atom Matom (atom %) (atom %) (atom %) (atom %) (atom %) (atom %) Example 1 Ga8.2 8.5 1.6 0.4 4.2 0 Comparative Ga 0.8 8.5 1.6 0.4 4.1 0 Example 1Example 2 Ga 1.2 8.4 1.6 0.4 4.2 0 Example 3 Ga 10.1 8.5 1.6 0.4 4.2 0Example 4 Ga 14.7 8.5 1.6 0.5 4.2 0 Comparative Ga 15.6 8.4 1.7 0.4 4.20 Example 2 Comparative Ga 8.2 8.5 1.6 0.4 4.2 0 Example 3 Example 5 Ga8.2 8.4 1.6 0.4 4.2 0 Example 6 Ga 8.2 8.5 1.6 0.4 4.2 0 Example 7 Ga8.2 8.4 1.6 0.5 4.2 0 Example 8 Ga 8.2 8.5 1.6 0.4 4.2 0 Comparative Ga8.2 8.5 1.6 0.4 4.2 0 Example 4 Comparative Ga 8.3 6.5 4.2 0 0 0 Example5 Example 9 Ga 8.2 6.4 4.3 0 0 0 Example 10 Ga 8.2 8.1 1.6 0.4 10.1 0Comparative Ga 8.2 8.1 1.6 0.4 21.0 0 Example 6 Example 11 Ga 8.2 7.91.6 0.4 10.0 10.0 Example 12 Ga 8.2 7.9 1.6 0.4 10.0 10.0 Electro-Crystal- Average magnetic lization particle conversion temperature sizeHc σs charac- (° C.) (nm) (Oe) (kA/m) (A · m²/kg) teristics Example 1640 16.3 2398 191 50 5 Comparative 640 16.2 2336 186 52 2 Example 1Example 2 640 16.4 2412 192 51 4 Example 3 640 16.3 2371 189 49 4Example 4 640 16.2 2366 188 46 4 Comparative 640 16.2 2455 195 41 2Example 2 Comparative 625 9.8 2012 160 48 2 Example 3 Example 5 627 10.42078 165 50 4 Example 6 649 21.0 2398 191 48 5 Example 7 643 17.4 2471197 49 5 Example 8 646 19.5 2679 213 49 4 Comparative 660 25.3 2884 23048 1 Example 4 Comparative 638 15.1 1940 154 48 2 Example 5 Example 9640 15.3 2070 165 50 4 Example 10 670 15.7 3950 314 48 4 Comparative 68016.6 4064 323 43 2 Example 6 Example 11 655 11.1 2543 202 47 4 Example12 675 15.4 3887 309 42 4

TABLE 1-2 Content of each constituent atom (with respect to 100.0 atom %of Fe atom) Kind of M atom Sr atom Ba atom Ca atom Al atom Nd atom Matom (atom %) (atom %) (atom %) (atom %) (atom %) (atom %) Example 13 Sc8.1 8.4 1.6 0.4 4.2 0 Example 14 Sc 1.1 8.4 1.6 0.4 4.2 0 Example 15 Sc14.3 8.4 1.7 0.4 4.2 0 Example 16 In 8.3 8.4 1.6 0.4 4.2 0 Example 17 In1.2 8.5 1.6 0.4 4.2 0 Example 18 In 14.6 8.4 1.6 0.4 4.2 0 Example 19 Sb8.4 8.4 1.6 0.4 4.2 0 Example 20 Sb 1.1 8.4 1.6 0.4 4.2 0 Example 21 Sb14.8 8.4 1.6 0.4 4.2 0 Example 22 Ga 8.2 9.1 0.0 0.0 4.2 0 Example 23 Ga8.2 8.5 1.6 0.4 0 0 Electro- Crystal- magnetic lization Averageconversion temperature particle Hc σs charac- (° C.) size (nm) (Oe)(kA/m) (A · m²/kg) teristics Example 13 640 15.6 2415 192 48 3 Example14 640 15.7 2261 180 46 3 Example 15 640 15.5 2443 194 42 3 Example 16640 16.0 2378 189 49 3 Example 17 640 16.1 2283 182 51 3 Example 18 64016.1 2411 192 41 3 Example 19 640 16.2 2359 188 48 3 Example 20 640 15.92397 191 50 3 Example 21 640 16.0 2299 183 42 3 Example 22 637 16.5 2277181 50 4 Example 23 618 16.4 2144 171 52 4

From the results shown in Table 1, it can be confirmed that, in themagnetic tapes of the examples, the electromagnetic conversioncharacteristics were excellent compared to those in the magnetic tapesof the comparative examples.

One aspect of the invention is useful in the technical field of amagnetic recording medium for high-density recording.

What is claimed is:
 1. A hexagonal strontium ferrite powder, wherein anaverage particle size is 10.0 to 25.0 nm, a content of one or more kindsof atom selected from the group consisting of a gallium atom, a scandiumatom, an indium atom, and an antimony atom is 1.0 to 15.0 atom % withrespect to 100.0 atom % of an iron atom, and a coercivity Hc is greaterthan 2,000 Oe and smaller than 4,000 Oe.
 2. The hexagonal strontiumferrite powder according to claim 1, which is a ferromagnetic powder formagnetic recording.
 3. The hexagonal strontium ferrite powder accordingto claim 1, wherein a mass magnetization as is equal to or greater than41 A·m²/kg.
 4. The hexagonal strontium ferrite powder according to claim1, wherein the content of the one or more kinds of atom selected fromthe group consisting of a gallium atom, a scandium atom, an indium atom,and an antimony atom is 1.0 to 12.0 atom % with respect to 100.0 atom %of an iron atom.
 5. The hexagonal strontium ferrite powder according toclaim 1, comprising: a gallium atom.
 6. The hexagonal strontium ferritepowder according to claim 4, comprising: a gallium atom.
 7. Thehexagonal strontium ferrite powder according to claim 1, comprising: ascandium atom.
 8. The hexagonal strontium ferrite powder according toclaim 4, comprising: a scandium atom.
 9. The hexagonal strontium ferritepowder according to claim 1, comprising: an indium atom.
 10. Thehexagonal strontium ferrite powder according to claim 4, comprising: anindium atom.
 11. The hexagonal strontium ferrite powder according toclaim 1, comprising: an antimony atom.
 12. The hexagonal strontiumferrite powder according to claim 4, comprising: an antimony atom. 13.The hexagonal strontium ferrite powder according to claim 1, furthercomprising: an aluminum atom.
 14. The hexagonal strontium ferrite powderaccording to claim 4, further comprising: an aluminum atom.
 15. Thehexagonal strontium ferrite powder according to claim 1, furthercomprising: a neodymium atom.
 16. The hexagonal strontium ferrite powderaccording to claim 4, further comprising: a neodymium atom.
 17. Thehexagonal strontium ferrite powder according to claim 1, wherein thehexagonal strontium ferrite powder has a magnetoplumbite type crystalstructure.
 18. A magnetic recording medium comprising: a non-magneticsupport; and a magnetic layer including a ferromagnetic powder and abinding agent on the non-magnetic support, wherein the ferromagneticpowder is the hexagonal strontium ferrite powder according to claim 1.19. A magnetic recording and reproducing apparatus comprising: themagnetic recording medium according to claim 18; and a magnetic head.